1. Field of the Invention
The present invention relates generally to adjusting the air/fuel ratio in the cylinders of an internal combustion engine to control automotive emissions. More particularly, the present invention relates to a method and system for adjusting the air/fuel ratio in the cylinders based on the amount of oxidants stored in the catalytic converter.
2. Background of the Invention
To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle. These emission control devices store oxygen and NOx (collectively, xe2x80x9coxidantsxe2x80x9d) from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, they release the stored oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.
The inventors have recognized a disadvantage with conventional air-fuel ratio control systems. In particular, the inventors have recognized that these systems attempt to maintain the engine at stoichiometry (or another desired air-fuel ratio). However, this has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the emission control device. The convention system relies on air-fuel feedback to compensate for this oversight.
The inventors herein have recognized that these known methods of adjusting cylinder air/fuel ratios, while effective, can be improved. In particular, the inventors have recognized that using the conventional air-fuel ratio control strategies tends to be reactionary in nature. That is, the cylinder air/fuel ratio tends to be adjusted more rich only after the exhaust stream oxygen sensors detect a NOx breakthrough. Similarly, the cylinder air/fuel ratio tends to be adjusted more lean only after the exhaust stream oxygen sensors detect hydrocarbon breakthrough. The inventors have recognized that a new method and system of controlling the engine air/fuel ratio to optimize catalyst efficiency that is anticipatory rather than reactionary would be beneficial.
The above disadvantage is overcome by a method for controlling an engine coupled to an emission control device. The method comprises determining an amount of oxidants stored in the emission control device; and adjusting a fuel injection amount into the engine to prevent said amount of stored oxidants from becoming less than a first predetermined level or from becoming greater than a second predetermined level.
By preventing the amount of stored oxidants from becoming less than a first predetermined level or from becoming greater than a second predetermined level, there is always a present ability to both:
(1) retain addition oxidants in the event the engine air-fuel ratio is inadvertently lean of stoichiometry; and
(2) reduce addition HC or CO in the event the engine air-fuel ratio is inadvertently rich of stoichiometry.
Thus, rather than controlling the engine air/fuel ratio around stoichiometry per se, as in the prior art, the present invention controls the engine air/fuel ratio to maintain a certain range of oxidants stored in the catalytic converter. That is, in one example, if the actual amount of oxidants stored in the catalyst at a given time is greater than a threshold, then the controller adjusts the engine air/fuel ratio more rich to produce hydrocarbons and release (and reduce) some of the oxidants from the catalyst. On the other hand, if the actual amount of oxidants stored in the catalyst at a given time is less a threshold, then the controller adjusts the engine air/fuel ratio more lean to produce additional NOx and O2 and replenish the relatively low amount of oxidants stored in the catalyst.
An advantage of the above aspect of the invention is improved overall catalyst efficiency and reduced emissions.